The present invention relates to a pumping set controlled by its rotary speed i.e. r.p.m., comprising an electric motor and a rotary pump driven thereby the rotary speed n of which may be regulated as a function of selected operating parameters of the set, in steps within the ambit of a characteristic range, the limits of which are determined on the one hand by the two modulation graphs H(Q) for the maximum and minimum r.p.m. figures and on the other hand by the co-ordination between delivery head H and delivery flow Q.
The point of operation of a pumping set as known coincides with the point of intersection of the plant characteristic H.sub.A (Q) and the modulation graph of the pump H(Q). Changes of this point of operation consequently render it necessary to vary the plant characteristic, the pump characteristic or both.
Varying the plant characteristic by restriction of a fitting or by opening of a by-pass, leads to power losses, as known. By contrast, the adaptation of the pump characteristic to the required operating condition of the set by r.p.m. variation, can be performed practically without loss. Apart from the lesser expenditure of power in such case, it is advantageous in many cases moreover for the r.p.m.-controlled pump to supply no more than the pressure difference required by the plant, so that flow noises may be averted. The plant outfitter thus requires a pumping set whose modulation graph may largely be adapted with satisfactory efficiency to the different plant characteristics and which may moreover be installed in as simple a manner as possible.
The requirement for pumps whose characteristics may be plotted at will has consequently already been put forward and discussed, the aim being to operate the pump outside the characteristic span range as far as possible, since flow noises in the water-carrying system and a poor control action could otherwise be expected.
Pumps developed under consideration of this principle, and the a.c. squirrel-cage motor of which may be operated under stepless control of r.p.m. by means of a frequency transofrmer, are available on the market. The pressure differential generated by the pump and measured, and the volumetric flow also measured, are compared in this case to a preprogrammed set graph and adjusted to this graph by acting on the r.p.m. Pumping sets controlled in this manner are very costly however, because of the involvement of mensuration techniques. Their control system is complex moreover and very vulnerable because of the considerable plant complexity. These sets are consequently limited to considerably powers as a rule and have to be installed by trained personnel.
It is an object of the invention to provide an inexpensive and uncomplicated r.p.m.-controlled pumping set, the plant graph of which may be in principle be optionally selected. This graph should be obtainable during operation of the set by stepped r.p.m. switching in optimum degree, without required complex mensuration techniques.